An examination of the helicopter and control systems did not reveal any mechanical discrepancies which would have caused a reduction or loss of tail rotor authority. Pilot and witness information indicates that the right yaw was not violent, as would be expected with a tail rotor system malfunction or failure. The pilot was aware of the low-speed flight characteristics of single rotor helicopters, which can result in an unanticipated right yaw, and the helicopter was flown in a manner which was conducive to a reduction of tail rotor authority. When the uncommanded right yaw occurred, the pilot applied full left pedal, but the helicopter continued to spin. Just prior to touchdown, the pilot did prepare for an autorotation, and this action may have contributed to the helicopter remaining upright, without major airframe breakup and probable injuries.Analysis An examination of the helicopter and control systems did not reveal any mechanical discrepancies which would have caused a reduction or loss of tail rotor authority. Pilot and witness information indicates that the right yaw was not violent, as would be expected with a tail rotor system malfunction or failure. The pilot was aware of the low-speed flight characteristics of single rotor helicopters, which can result in an unanticipated right yaw, and the helicopter was flown in a manner which was conducive to a reduction of tail rotor authority. When the uncommanded right yaw occurred, the pilot applied full left pedal, but the helicopter continued to spin. Just prior to touchdown, the pilot did prepare for an autorotation, and this action may have contributed to the helicopter remaining upright, without major airframe breakup and probable injuries. The aircraft systems were examined to the degree possible, and no evidence of a malfunction was found. Bell Helicopter Textron Inc. has previously identified conditions under which all single rotor helicopters can encounter unanticipated right yaw from reduction of tail rotor effectiveness. The pilot had considerable experience as a helicopter pilot and instructor, and acknowledged the information in AC90-95, but did not recognize the situation developing prior to the loss of control.Findings The aircraft systems were examined to the degree possible, and no evidence of a malfunction was found. Bell Helicopter Textron Inc. has previously identified conditions under which all single rotor helicopters can encounter unanticipated right yaw from reduction of tail rotor effectiveness. The pilot had considerable experience as a helicopter pilot and instructor, and acknowledged the information in AC90-95, but did not recognize the situation developing prior to the loss of control. The pilot lost control of the helicopter because of an unanticipated right yaw caused by a reduction in tail rotor authority.Causes and Contributing Factors The pilot lost control of the helicopter because of an unanticipated right yaw caused by a reduction in tail rotor authority. Transport Canada has indicated that information on loss of tail rotor effectiveness could be presented in Aviation Safety Vortex. An article describing the conditions leading up to the sudden right yaw and loss of control is planned for an early issue in 1999.Safety Action Transport Canada has indicated that information on loss of tail rotor effectiveness could be presented in Aviation Safety Vortex. An article describing the conditions leading up to the sudden right yaw and loss of control is planned for an early issue in 1999.